Technical Field
The present invention relates to a method for producing a rare-earth magnet.
Background Art
Rare-earth magnets that use rare-earth elements are also called permanent magnets. Such magnets are used not only for hard disks or motors of MRI but also for driving motors of hybrid vehicles, electric vehicles, and the like.
As examples of magnetic performance indices of such rare-earth magnet, remanent magnetization (i.e., residual magnetic flux density) and coercivity can be given. However, with a reduction in the motor size and an increase in the amount of heat generation accompanied by an increase in the current density, there has been an increasing demand for higher heat resistance of the rare-earth magnet being used. Thus, how to retain the coercivity of a magnet under high-temperature use environments is an important research object to be achieved in the technical field.
For example, for a Nd—Fe—B-based magnet, which is one of the rare-earth magnets that are frequently used for vehicle driving motors, attempts have been made to increase the coercivity by, for example, reducing the crystal grain size, using an alloy with a high Nd content, or adding a heavy rare-earth element with high coercivity performance, such as Dy or Tb.
Examples of rare-earth magnets include typical sintered magnets whose crystal grains that form the structure have a scale of about 3 to 5 μm, and nanocrystalline magnets whose crystal grain size has been reduced down to a nano-scale of about 50 to 300 nm.
In order to increase the coercivity, which is one of the magnetic properties, of a rare-earth magnet, Patent Document 1 discloses a method of modifying a grain boundary phase by, for example, diffusing and infiltrating a Nd—Cu alloy or a Nd—Al alloy into the grain boundary phase, as a modifying alloy that contains a transition metal element and a light rare-earth element.
Such a modifying alloy that contains a transition metal element and a light rare-earth element has a low melting point as it does not contain a heavy rare-earth element, such as Dy. Thus, the modifying alloy melts at about 700° C. at the highest, and thus can be diffused and infiltrated into the grain boundary phase. Therefore, for a nanocrystalline magnet whose crystal grain size is less than or equal to about 300 nm, such a method is said to be a preferable processing method as it can improve the coercivity performance by modifying the grain boundary phase while at the same time suppressing coarsening of the nanocrystal grains.
By the way, in order to improve the magnetization of a rare-earth magnet, attempts have been made to increase the proportion of the main phase (e.g., to about 95% or greater). However, when the proportion of the main phase is increased, the proportion of the grain boundary phase will decrease correspondingly. Therefore, when a modifying alloy is diffused in the grain boundaries in such a case, a problem may occur such that the molten modifying alloy cannot sufficiently infiltrate the inside of the rare-earth magnet, resulting in decreased coercivity performance, though the magnetization improves.
For example, even Patent Document 1 does not deal with such a problem, and thus fails to disclose means for solving the problem.